It's not so easy to get information about the core of photosynth (How all the steps are done).
We all have ideas but without real veracity.
The more interesting information could be found on http://getsatisfaction.com/livelabs a forum where microsoft guys answer to customer questions.
So on the question how photosynth compute FOV values (aka Field of view). It's for sure from exif data but with a little help. If the camera is known they use a internal database of CDD sensor width. One years ago they claims to get 600 camera reference in their database. (I do not imagine how many they have today).
So from this information and test we could deduce that Photosynth is not a autocalibration process (If they do not find the FOV they use an arbitrary value).
At your turn could you find more information ?
Source : GetStatisfaction
"We have a database with sensor sizes for about 600 cameras in the product now. We combine the sensor size with the focal length in the EXIF to compute the Field of View (FOV). Why do we need this? Imaging taking two photos of the same scene that look identical, one taken very near with wide angle and one taken further away with telephoto. By computing the FOV from the lens/camera combination we have a more accurate estimate of distance which improves the overall output of the synth. If we don't know the FOV we use a default starting point and the synther actually figures out what the FOV was as it pieces things together. This usually works fine but loosely connected synths do benefit from having this knowledge ahead of time.
Right now the sensor sizes are baked into the synther."